The present invention relates to a two-stage pump for use in a continuous ink jet printing system.
In our copending British application number 9009957.3 there is disclosed an ink jet supply system for a continuous ink jet printer in which ink under pressure is fed through a nozzle, the ink stream being broken up into small droplets which are then individually charged and deflected in order to print on a substrate moving relative to the nozzle. The present invention relates to a pump which is suitable for use in such a system.
In a continuous ink jet printer, drops of ink are propelled at typical speeds of 18 to 25 ms.sup.-1 over a distance of typically 75 mm. This is achieved by pressurising ink at a constant pressure within a chamber which opens into a nozzle of typical orifice size 60 .mu.m. Pressure is applied to the ink either by direct pumping of the ink, and then regulating that pressure to the required level, or by indirect means such as pressurising the air above the ink reservoir. Unprinted ink drops are returned to the bulk of the ink via a drop catcher and gutter, through which air flows in order to move the ink along the gutter.
Known devices for applying pressure to the ink include a gear pump, in which the resulting pressure generated is usually pulsed due to the action of the gear teeth, necessitating the use of some form of damping; a compressor, in which pressure is applied to the surface of an ink reservoir, ink being transferred from a low pressure side by a separate transfer system; a peristaltic pump which is a low pressure pump frequently used for drawing ink and air down a gutter line prior to transfer to a high pressure pump; and a positive displacement pump which may operate with or without a diaphragm. When an ink jet printer is turned on, a higher flow of air is required down the gutter than is needed when unprinted drops are present in the gutter line. It is common, therefore, to design the pump to deliver a higher air flow rate than that required when the system is printing. This feature results in a high rate of ink solvent usage due to evaporation. Furthermore, it is common to direct a bleed line from the printhead to the most convenient source of low pressure, which often happens to be the gutter line, which reduces the gutter efficiency whilst the bleed is in operation. Moreover, continuous ink jet printers are often used in industrial environments, placing heavy demands on the machine, and pumps, such as gear pumps, operate at very high cycle speeds typically in excess of 1000 rpm and are thus prone to rapid wear and early failure.